[1] He accepted a position at the National Print Cabinet in The Hague, where he began work on a German concept of image-based historical research, which due to the special circumstances of the interbellum period was drastically reduced.

He eventually finished his PhD thesis 12 July 1940, cum laude, on the patriotic subject of Zeventiende eeuwsche uitbeeldingen van den Bataafschen Opstand (17th-century images of the Batavian Revolt).

In 1946 he presented his ideas about mapping the iconography of art history with "beeld-leer", an image-based concept of recording form, function and content with one code.

[4][5] Among the same collection presently at Leiden University, van de Waal also noted "Man arriving in a village, a sheep on his shoulders, Old Testament scene?"

[7] Jaap Hillenius exchanged letters with Van de Waal, and one such is among the collection of the Library of Linden University with a drawing of a standing man on the wrong side.